Fragilaria is a genus of diatoms (Bacillariophyta) with about 100 species found in freshwater and seawater. It is often abundant in the plankton of lakes. Fragilaria crotonensis (like Astrionellla Formosa) is a species that is considered to have a worldwide distribution and is also considered to be introduced by human activities.
The cells in Fragilaria are arranged in flat, linear tapes which are often rotated in its longitudinal axis. The individual cells have the typical diatom shell of two covers. This is rectangular in side view, in shell view it is rod-shaped. The cells have a central nucleus and two plastids that are under the shell surface, and are colored golden brown by fucoxanthin. The plastids contain several pyrenoids. Some species have more than one disc-shaped plastids. Since the cells in the tapes are always connected to each other by the shell surfaces, the side view will be seen almost always.
Asexual reproduction occurs by the typical dichotomy of diatoms, which leads to the extension of the tape. The tape can also break. The formation of spores is known, whether they are formed through sexual reproduction is not known. The variability of the species results from different cell sizes, different length to width ratios of the cells in side view and the shape of the cells in the shell view.